The present invention relates to an improved method for preparing (xcex1-aspartyl)-xcex2-cyclohexylalaninamide from an ester of (xcex1-aspartyl)phenylalanine.
(xcex1-Aspartyl)phenylalaninamide is usually prepared by peptide coupling of derivatives of aspartic acid and of phenylalaninamide. These couplings require protective operations to be carried out on the aspartic acid, and the phenylalaninamide has to be prepared from phenylalanine. After peptide coupling, amine and acid deprotective operations are needed to gain access to (xcex1-aspartyl)phenylalaninamide, thus rendering its synthesis lengthy, inefficient and expensive [J. Org. Chem., 40, 2495 (1975); WO 9006937;DD 209 191; DE 2 245 459; EP 149 582].
Patent Application WO 95/10295 has disclosed the preparation of pseudotetrapeptide derivatives of general formula: 
in which in particular Z represents: 
for which E is in particular hydrogen, F is in particular cycloalkylalkyl, r can be 1 and G can form an NR1R2 group.
These derivatives, which are active in the cardiovascular field, are prepared via aspartic derivatives, such as, for example, the aspartic derivative of general formula: 
However, the process is fairly lengthy and expensive, since it involves starting materials, such as cyclohexylalanine and the monobenzyl ester of benzyloxycarbonylaspartic acid, which are not commercially available, in particular on an industrial scale, which starting materials have to be protected beforehand before they are employed in the process. Thus, the industrial preparation of the biologically active product is extremely laborious.
Patent Application EP 405 506 has disclosed the preparation of (xcex1-aspartyl)cyclohexylalaninamide by catalytic hydrogenation of (xcex1-aspartyl)phenyl-alaninamide. However, (xcex1-aspartyl)phenylalaninamide prepared by conventional methods could not make it possible to obtain an overall improvement in the method.
It has now been found, and it is this which forms the subject-matter of the present invention, that it is possible to prepare the ammonium salt of (xcex1-aspartyl)phenylalaninamide of formula: 
directly from an ester of (xcex1-aspartyl)phenylalanine of general formula: 
in which R is an alkyl radical comprising 1 to 4 carbon atoms, and that it is possible, for this reason, to prepare (xcex1-aspartyl)-xcex2-cyclohexylalaninamide or its salt in only 2 stages.
According to the invention, the ammonium salt of (xcex1-aspartyl)phenylalaninamide is prepared by amidation of the ester of (xcex1-aspartyl)phenylalanine of general formula (II) and then, in a second stage, (xcex1-aspartyl)-xcex2-cyclohexylalaninamide is obtained by catalytic hydrogenation of the product obtained, optionally released beforehand from its salt (optionally in situ).
According to a preferred form of the invention, the method is carried out on the ester of ((L)-xcex1-aspartyl)-(L)-phenylalanine in order to prepare ((L)-xcex1-aspartyl)-(L)-phenylalaninamide and then ((L)-xcex1-aspartyl)-(L)-xcex2-cyclohexylalaninamide.
Also according to a preferred form, the invention can be carried out starting from the methyl ester of (xcex1-aspartyl)phenylalanine.
According to the invention, the amidation reaction is carried out by reaction with liquid ammonia at a temperature of between xe2x88x9240 and 20xc2x0 C. It can optionally be carried out in the presence of a cosolvant, such as water, an alcohol (methanol, ethanol, isopropanol or ethylene glycol) or acetonitrile.
When it is desired to release the product from its salt, in order to obtain ((L)-(xcex1-aspartyl)-(L)-phenylalaninamide as an intermediate, the ammonium salt obtained above can either be directly heated for 12 to 16 hours at a temperature of between 30 and 40xc2x0 C. while flushing with nitrogen and optionally under reduced pressure or can be treated in an aqueous acidic medium as described below in the examples, in particular with acids as described below.
When it is desired to release the product from its salt and to obtain an acid salt, the ammonium salt is treated in an acidic medium, according to the usual methods which do not detrimentally affect the remainder of the molecule, as described below in the examples, in particular, and without implied limitation, with acids such as hydrochloric acid, tartaric acid, acetic acid, oxalic acid, lactic acid, citric acid or mandelic acid.
The catalytic hydrogenation stage is carried out at a temperature of between 20 and 60xc2x0 C. (preferably at 40xc2x0 C.) under a hydrogen pressure of 1 to 8 bar (preferably 4 bar) in an aqueous hydrochloric acid medium or in an acetic acid medium in the presence of platinum and optionally in the presence of another organic acid, such as, for example, trifluoroacetic acid, trichloroacetic acid, oxalic acid, malonic acid, citric acid, tartaric acid, malic acid, formic acid or lactic acid.
The derivatives obtained by the method according to the invention can optionally be converted to addition salts with acids.
Mention may be made, among industrially advantageous salts, of the hydrochloride, hydrobromide, tartrate, acetate, oxalate, lactate, citrate, mandelate or trifluoroacetate.
The method according to the invention is particularly advantageous owing to the fact that it opens the route to the preparation of (xcex1-aspartyl)-xcex2-cyclohexylalaninamide in only 2 stages and also owing to the fact that it makes it possible to prepare the intermediate amide of formula (I) in a single stage from a starting material which is readily accessible industrially: aspartame.
The (xcex1-aspartyl)-xcex2-cyclohexylalaninamide thus obtained can be purified according to the usual methods, such as chromatography or crystallization.
(xcex1-Aspartyl)-xcex2-cyclohexylalaninamide can be employed in the preparation of pseudotetrapeptide derivatives by carrying out the preparation according to the method disclosed in International Application WO 95/10295.